Mechanism of bioleaching of coal fly ash by Thiobacillus thiooxidans

Bioleaching of aluminum and iron from coal fly ash (CFA) by Thiobacillus thiooxidans ( T. thiooxidans) bacteria is considered. The interactions between bacteria, metabolic products, CFA particles, and leaching products were studied. It is demonstrated that bacterial growth and the amount of metals leached from the CFA are coupled through biological and chemical interactions, which involve the various components in this system. Bioleaching experiments were performed batch wise by suspending up to 10% (w/v) CFA in T. thiooxidans growth medium containing cell inoculum for a typical 3 week period of time. Samples were taken periodically from leached suspensions and relevant parameters including metals’ concentrations, cell counts, pH and extracellular polymeric substances (EPS) were determined. The results show that under the same conditions, similar leaching levels are obtained by sulfuric acid and bioleaching of CFA, and the contribution of other metabolites is insignificant. CFA inhibits the growth rate through two major effects. The first is due to the alkaline components released by the CFA that cause a rise in the pH, and a corresponding delay in growth. The second is attributed to the random attachment of the bacteria to both the sulfur particles (the energy source) and the barren CFA particles, resulting in a so-called “dilution effect” of the sulfur particles, and an inhibition of the initial growth rate. However, after an adaptation period of the bacteria the subsequent growth rate, the maximal cell concentration and minimal pH were similar to those obtained in the control experiment, irrespective of CFA content. Enhanced excretion of EPS was observed in the presence of CFA as well as in calcium and barium enriched growth media. It is presumed that the mechanism of EPS production is related to the presence of the particulate solid phase.